The invention relates generally to thermoplastic reworkable encapsulants for integrated circuit assemblies. More specifically, the invention relates to reworkable encapsulants for integrated circuits which comprise high molecular weight polymers synthesized by the ring opening polymerization of low molecular weight cyclic oligomers.
Integrated circuit components are often encapsulated to maintain structural and functional integrity. In many instances, the encapsulant is a cured polymer. The attributes of curable polymers include the combination of processability prior to curing with outstanding properties after curing. Curable resins generally have a low viscosity prior to curing, even in the absence of solvents. After curing, these polymer compositions exhibit thermal stability, toughness, adhesion, and solvent resistance.
The attributes of these polymer compositions also may include intractability after curing. This intractability is often the result of a curing reaction in a thermosetting resin which converts a low molecular weight precursor to a polymer network of essentially infinite molecular weight. These attributes make thermosetting resins ideal for use in the construction of circuit assemblies such as single-sided and double-sided circuits, as well as other types of surface mount technology including chip carriers, multichip modules and multilayer boards.
Exemplary compositions which have been disclosed as useful in encapsulating semiconductor devices include those taught by Eickman, et al., U.S. Pat. No. 4,632,798. Eickman, et al. disclose a molding composition comprising a melt processable thermotropic liquid crystal and polymer having a relatively low weight average molecular weight of about 4,000 to about 10,000 which is substantially incapable of further chain growth upon heating. Dispersed within the liquid crystal and polymer is approximately 40-80% by weight of a particular inorganic material (preferably silicon dioxide). Eickman, et al. address problems with prior encapsulants including the need for refrigeration prior to use and require relatively long cycle and cure times during molding.
Christie, et al., U.S. Pat. No. 5,250,848, address the solder connections used for joining an integrated semiconductor device to a carrier substrate and particularly to a stricture for forming solder interconnection joints that exhibit improved fatigue life and stability. The Christie, et al. encapsulant composition contains a binder such as a cycloaliphatic polyepoxide, an inorganic filler, and optionally a surfactant.
Papathomas, et al., U.S. Pat. No. 5,471,096, discloses a method of increasing the fatigue life of solder interconnections between a semiconductor device and a supporting substrate. The Papathomas composition contains about 40 to 70% by weight bisphenyl M dicyanate and preferably 30% by weight of the 4, 41-ethylidene bisphenol dicyanate, and an inorganic filler. The compositions can also include a catalyst to promote the polymerization of the cyanate ester mixture.
Kohler, et al., U.S. Pat. No. 5,064,895, also teaches molding compounds of polyarylene sulfide (PAS), preferably polyphenylene sulfides (PPS) which have a low iron content and delayed crystallization and to their use as an encapsulating compound for both active and passive electronic components.
Nakai, U.S. Pat. No. 5,160,786, discloses a resin material for inserting a lead frame of a polyarylene sulfide resin; an alpha olefin copolymer graft copolymerized with an unsaturated carboxylic acid or its anhydride; and a fibrous filler, a nonfibrous inorganic filler or a mixture thereof.
However, once cured, these encapsulants form non-reworkable and intractable masses. The intractability of encapsulants has become more of a liability because of concerns about the longevity of circuit assemblies in the environments of use. Additionally, in many applications, the attachment of discrete devices, and their subsequent encapsulation is a continuing process which often requires the application of high temperatures in the circuit assembly environment. These high temperatures can char and destroy a cured encapsulant. Also, many manufacturers are taking responsibility for disposal or recycling of their products. Manufacturers may even be required to be responsible for disposal or recycling of products through government regulation.
Intractable compositions are also not compatible with the concept of disassembly for purposes of repair, disposal, or recycling, regardless of whether the thermosets are used as structural components, adhesives, or encapsulants. If, however, the compositions are designed for disassembly, it is possible that the many advantages of the thermosets can be retained without the disadvantages of intractability.
As a result, there is a need for encapsulants which provide the requisite curing properties and physical stability once cured which are at the same time reworkable so as to allow for the repair and replacement of discrete devices in the environment of an integrated circuit assembly.
In accordance with a first aspect of the invention, there is provided an encapsulated circuit assembly comprising: a chip; a substrate; at least one solder joint which spans between the chip and the substrate forming an electrically conductive connection between the chip and the substrate; and an encapsulant formed adjacent the solder joint, wherein the encapsulant comprises a thermoplastic polymer formed by ring opening polymerization of a cyclic oligomer.